Geographic Range

River blackfish (Gadopsis marmoratus) have a relatively limited range, inhabiting freshwater rivers in southeastern Australia. They are found both north and south of the Great Dividing Range, a mountainous geographic barrier that divides Victoria into two separate regions. (Barnham, 2007)

Habitat

Although river blackfish can be found in both slower and faster flowing waters, they prefer to stay in low-velocity (0 to 20 cm/s), highly sheltered pools of lowland rivers. Shelter in the pools includes rocks and woody debris, along with other inputs from the terrestrial landscape, which help to slow the flow of the river. The waters that the fish inhabit are variable in temperature, ranging from 5 to 28 °C, and contain a large amount of aquatic vegetation. Tagging studies have shown that adult river blackfish swim at deeper depths relative to stream surface (40 to 60 cm) and that depth preference is positively correlated with the size of the individual fish. Oxygen-rich bodies of water are most suitable for river blackfish survival.
Smaller, immature blackfish are usually not found at deeper depths due to the predation risk posed by larger carnivorous fish such as adult brown trout (Salmo trutta). These immature fish tend to remain in shallow waters, burrowing themselves under leaf litter. (Dobson and Baldwin, 1982a; Dobson and Baldwin, 1982b; Jackson, 1978a; Jackson, 1978b; Khan, et al., 2004)

Physical Description

Average length and weight are 30 cm and 450 g, respectively. Body size varies geographically in river blackfish. River blackfish in southern Victoria are significantly larger than those found in northern Victoria on average. They also show great diversity in terms of dorsal and ventral body color. Lateral and dorsal coloration is variable and can be mottled yellow, brownish-green, grey, dark brown, or light blue. These background colors are peppered with many dark, irregular blotches. Ventral color may be light blue, yellow, or purple.

River blackfish are elongated and round, and the body is covered in small scales. The dorsal and anal fins are long and found low on the body. The fish is covered in a thick layer of slime, a characteristic which has earned river blackfish alternative nicknames such as slippery or slimy blackfish. River blackfish have muscle composed almost entirely of white muscle, which is especially conducive to short, quick, burst movement, rather than consistent steady swimming. Although the abundance of white muscle is extremely beneficial in terms of capturing prey, the low endurance that results from a severe lack of red muscle reduces the ability of river blackfish to escape natural predators and human anglers.

River blackfish have hemoglobin in their blood with low oxygen affinity relative to that of other aquatic fish. However, they exhibit thermal acclimation, which allows them to adjust their blood’s oxygen affinity depending on temperature. Fish acclimated to temperatures of 20°C have hemoglobin that favors increased release of oxygen to muscle tissues, whereas fish acclimated to temperatures of around 10°C have hemoglobin that is more efficient at oxygen uptake from the surrounding environment. (Barnham, 2007; Dobson and Baldwin, 1982a; Dobson and Baldwin, 1982b)

Development

Research on river blackfish development has only focused on early development. However, there is no evidence to suggest that they develop in a radically different manner from other bony fish. After they are fertilized externally, the eggs develop for about 14 days. After hatching, the offspring are in the "fry" stage of development. While fry, immature river blackfish carry a yolk sack for nutrient absorption that remains attached to their body until their digestive system finishes developing. The fry stage lasts for approximately 26 days. For 19 of these days, fry remain bound to the substrate upon which they were deposited as eggs. For the last seven days, the fry are able to wiggle free from the substrate and swim freely. River blackfish express none of their characteristic pigmentation while in the fry stage. Pigmentation develops later and is seen to its fullest extent once the fish reach the juvenile stage of development. (Jackson, 1978a; Jackson, 1978b)

Reproduction

River blackfish reproduce sexually, but their mating system is unknown. Fertilization of eggs occurs outside of the mother’s body, and eggs are normally laid inside hollow logs. Egg deposition has also been observed inside of man-made structures (e.g., hollow pipes), which may simulate the conditions of hollow logs. (Barnham, 2007; Jackson, 1978a; Jackson, 1978b)

The reproductive season starts in either late spring or summer (October to November in Australia) and is stimulated by water temperatures of 16°C or higher. River blackfish are known for their very low fecundity, averaging approximately 300 eggs per spawning season. This low fecundity is striking, especially when compared to fish such as salmon, which may lay as many as 7,500 eggs in a year. The overall number of eggs laid is positively correlated with the length of a female’s body. (Barnham, 2007; Jackson, 1978a; Jackson, 1978b)

Observations in natural and artificial environments demonstrate that males exhibit parental care in the form of protecting eggs until they hatch and fanning them to keep them well-oxygenated. Females have also been observed lingering around egg sites, suggesting the possibility of female parental investment as well. Parental investment is consistent with their low fecundity; with so few eggs parents must expend energy to ensure maximum survival of offspring. (Barnham, 2007; Jackson, 1978a; Jackson, 1978b)

Lifespan/Longevity

The lifespan of Gadopsis marmoratus has not been documented in nature or in captivity.

Behavior

River blackfish are nocturnal, solitary fish. They exhibit diel movement patterns, using different subsections of their river habitats depending on the time of day. At night, they tend to move to open areas with higher water velocities, during the day they move to low velocity, sheltered pools.
One explanation for the spending time in both fast moving and slow moving areas is that river blackfish cannot tolerate constant, fast moving currents due to their lack of red muscle cells. Staying in slower moving areas during the day allows the fish to conserve energy for the night, during which time they may scour their environment for prey items.
Predator avoidance and improved hunting ability may explain why river blackfish inhabit shelters areas during the night and venture into open areas during the night. If river blackfish spend too much time in the open during the day, they are vulnerable to visual predators. Camouflaging themselves in river shelter helps the fish to avoid predation during the day. However, most of their terrestrial predators are inactive at night, and river blackfish are not as easily seen under the cover of darkness. This makes venturing into the open safer at night for Gadopsis marmoratus than it is during the day.

In addition to protection, the darkness of night allows river blackfish to be more successful predators in open areas. During the day, it is not advantageous to expend energy in open, easily visible areas, as most of their hunting attempts will be easily detected and unsuccessful. Being an ambush predator, dark conditions allow river blackfish to successfully approach prey in open areas – prey that might otherwise detect them with adequate light. (Barnham, 2007; Dobson and Baldwin, 1982a; Dobson and Baldwin, 1982b; Jackson, 1978a; Jackson, 1978b; Khan, et al., 2004)

Home Range

River blackfish have a restricted home range. Throughout their lifespan they occupy an area of from 16 to 30 meters of a single stretch of river. They also show a high affinity for their established territory, as blackfish are usually found to return to their original positions following translocation. (Khan, et al., 2004; Koster and Crook, 2008)

Communication and Perception

No in-depth studies regarding communication and perception of river blackfish have been conducted. Like other fish, they are likely to use a combination of vision, chemosensation, and tactile cues to perceive their environment. (Khan, et al., 2004)

Food Habits

River blackfish are carnivorous, ambush predators. They prefer to ambush prey areas of cover in order to most efficiently use their short, quick bursts of speed. They have a diverse diet. Prey items include insects, mollusks, crustaceans, small fish, and terrestrial invertebrates that fall into the water. Of all their prey items, larval mayflies are most important, comprising 50% of the diet. Terrestrial insects make up the second greatest portion of the blackfish diet, 10% of their overall intake. (Barnham, 2007; Jackson, 1978a; Jackson, 1978b)

Predation

Anti-predator adaptations differ between large and small members of the species. Juvenile river blackfish are most susceptible to predation by large carnivorous fish found in deeper waters. In order to avoid this predation, smaller individuals tend to stay in shallow waters under the cover of leaf litter and aquatic plants. Young river blackfish spend much time under leaf litter and little time out in the open in order to avoid predation by dragonfly nymphs (Odonata) and freshwater crayfish (Parastacidae).

Larger blackfish are not prey to fish in deep waters. Instead they are the primary targets of larger, terrestrial predators that dive and wade for food, such as herons. Larger fish have limited mobility in the confined space of shallow waters, and they are more susceptible to terrestrial predation when they are exposed so close to the surface. Therefore, large blackfish tend to remain in deeper waters in order to minimize the risk of terrestrial predation. (Khan, et al., 2004)

Ecosystem Roles

River blackfish compete with introduced brown trout (Salmo trutta) for resources. These two fish species have similar diets, the main difference being that river blackfish consume significantly more caddisfly larvae. River blackfish control insect populations through predation. (Jackson, 1978a; Jackson, 1978b)

Economic Importance for Humans: Positive

River blackfish are a common angling fish in Australia and are considered excellent for human consumption. However, the restricted range of river blackfish, especially compared to game fish such as salmon and trout, prevents these fish from contributing to the economy on a larger scale. (Barnham, 2007)

Economic Importance for Humans: Negative

There are no known adverse effects of Gadopsis marmoratus on humans.

Conservation Status

River blackfish are common throughout their geographic range. However, due to human influence, river blackfish populations have been significantly reduced in certain areas. Competition with introduced trout and overfishing have put a strain on river blackfish, causing their numbers to decline. In addition, human influence has reduced stream debris (removing essential shelter), and construction has led to an increase in sedimentation, degrading aquatic habitats. Such an impact, along with their low fecundity, means that populations are vulnerable to endangerment or extinction and should be monitored closely in the future. (Barnham, 2007; Bond and Lake, 2005; Jackson, 1978a; Jackson, 1978b)

Contributors

Percy LeBlanc (author), The College of New Jersey, Keith Pecor (editor), The College of New Jersey, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

male parental care

parental care is carried out by males

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

vibrations

movements of a hard surface that are produced by animals as signals to others

Jackson, P. 1978. Spawning and early development of the river blackfish, Gadopsis marmoratus Richardson (Gadopsiformes : Gadopsidae), in the McKenzie River, Victoria. Australian Journal of Marine and Freshwater Research, 29: 293-298.

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This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.